Wheel bearing apparatus is used to freely rotationally support a wheel hub for mounting the wheel via a rolling bearing for a driving wheel and a driven wheel. For structural reasons, an inner ring rotation type is generally adopted for a driving wheel. Both inner ring rotation and outer ring rotation types are adopted for a driven wheel. Double row angular contact ball bearings are widely used in such a bearing apparatus for reasons that they have a desirable bearing rigidity, high durability against misalignment and small rotation torque to improve fuel consumption. The double row angular contact ball bearing has a structure with a plurality of balls that are interposed between a stationary ring and a rotational ring. Also, a predetermined contacting angle is applied to the balls relative to the stationary and rotational rings.
The bearing apparatus for a wheel of a vehicle is broadly classified into a first through fourth generation structure. In a first generation type, a wheel bearing with a double row angular contact ball bearing is fit between a knuckle forming part of a suspension and a wheel hub. In a second generation structure, a body mounting flange or a wheel mounting flange is directly formed on the outer circumference of an outer member. In a third generation structure, one of the inner raceway surfaces is directly formed on the outer circumference of the wheel hub. In a fourth generation structure, the inner raceway surfaces are directly formed on the outer circumference of the wheel hub and the constant velocity universal joint.
In prior art wheel bearing apparatus formed with a double row rolling bearing, since both bearing row arrangements are the same in the double row bearing, the apparatus has a sufficient rigidity during straight way running, however, optimum rigidity cannot always be obtained during curved way running. The positional relationship between the wheels and the bearing apparatus is usually designed so that the weight of the vehicle acts at substantially the center between the rows of bearing balls during the straight way running. However, a larger radial load and a larger axial load are applied to vehicle axles on the side opposite to the curving direction (i.e. axles of the left hand side of vehicle when right hand curving). Accordingly, it is effective to have a larger rigidity on the bearing row of the outer-side than that of the bearing row of the inner-side in order to improve the durability and strength of the bearing apparatus. Thus, a known vehicle wheel bearing apparatus is shown in FIG. 6 that can have a high rigidity without enlargement of the bearing apparatus. In the description below, the term “outer-side” (left hand side in the drawings) of the apparatus denotes a side that is positioned outside of the vehicle body. The term “inner-side” (right hand side in the drawings) of the apparatus denotes a side that is positioned inside of the body when the bearing apparatus is mounted on the vehicle body.
The vehicle wheel bearing apparatus 50 is formed by a double row angular contact ball bearing with an outer member 51 integrally formed on its outer circumference with a body mounting flange 51c to be mounted on a knuckle (not shown) of a vehicle. The outer member inner circumference has a double row outer raceway surfaces 51a, 51b. An inner member 55 includes a wheel hub 52 with a wheel mounting flange 53 integrally formed at one end for mounting a wheel (not shown). One inner raceway surface 52a is formed on the outer circumference of the wheel hub opposite to one 51a of the double row outer raceway surfaces 51a, 51b. A cylindrical portion 52b axially extends from the inner raceway surface 52a. An inner ring 54 is fit onto the cylindrical portion 52b. The inner ring is formed on its outer circumference with the other inner raceway surface 54a opposite to the other raceway surface 51b of the double row outer raceway surfaces 51a, 51b. Double row balls 56, 57 are freely rollably contained between the outer raceway surfaces 51a, 51b and inner raceway surfaces 52a, 54a of the inner member 55. Cages 58, 59 rollably hold the balls 56, 57.
The inner ring 54 is axially immovably secured by a caulked portion 52c. The caulked portion 52c is formed by radially outwardly plastically deforming the cylindrical portion 52b of the wheel hub 52. Seals 60, 61 are mounted in annular openings formed between the outer member 51 and the inner member 55. The seals 60, 61 prevent leakage of grease contained within the bearing apparatus and the entry of rain water or dusts into the bearing apparatus from the outside.
A pitch circle diameter D1 of the outer-side ball group 56 is set larger than a pitch circle diameter D2 of the inner-side ball group 57. Accordingly, the diameter of the inner raceway surface 52a of the wheel hub 52 is larger than the diameter of the inner raceway surface 54a of the inner ring 54. Also, the outer raceway surface 51a diameter of the outer-side of the outer member 51 is larger than the diameter of the outer raceway surface 51b of the inner-side of the outer member 51. Also, the number of outer-side balls 56 is larger than the number of the inner-side balls 57. By setting the pitch circle diameter D1 of the outer-side larger than the pitch circle diameter D2 of the inner-side (D1 D2), it is possible to obtain a large rigidity of the bearing apparatus 50 and thus extend its life. (Japanese Laid-open Patent Publication No. 108449/2004).
In prior art wheel bearing apparatus 50, the pitch circle diameter D1 of the outer-side ball group 56 is set larger than the pitch circle diameter D2 of the inner-side ball group 57. Accordingly, the diameter of the outer-side outer raceway surface 51a of the outer member 51 is formed larger than the diameter of the inner-side outer raceway surface 51b of the outer member 51. Additionally, these outer raceway surfaces 51a, 51b are formed with a hardened layer by high frequency induction quenching. This improves the rigidity of the outer-side bearing row. Thus, this assures the rolling fatigue life, strength and durability of the wheel bearing apparatus 50.
In the prior art wheel bearing apparatus, the inner member 55 has a moment load during driving of the vehicle via the wheel mounting flange 53. The inner raceway surface 52a suffers from a compressive stress from the balls 56 in addition to a bending stress caused by the moment load. Thus, the metal material forming the inner member 55 simultaneously suffers from a tensile stress caused by the bending stress and a shearing stress caused by the compressive stress. Accordingly, it is difficult to assure sufficient durability without a means for resisting these tensile stresses and shearing stresses.
Also in the wheel bearing apparatus, it is feared that impressions may be formed on the raceway surfaces, via the balls 56, when a vehicle rides over a curb. The impressions on the raceway surfaces would cause noise and shorten the fatigue life of the bearing apparatus.
In addition, any deformation would be caused on shoulders 62, 63 between the outer raceway surfaces 51a, 51b of the outer member 51 when the outer member 51 suffers from an excessive impact load. The shorter the pitch “P” between the double row balls 56, 57, the easier such a deformation may be caused. Accordingly, the hardened layers are usually formed not only on the double row outer raceway surfaces 51a, 51b but on the shoulders 62, 63 between the double row outer raceway surfaces 51a, 51b. However, not only do the manufacturing cost increase but accuracy is diminished due to the heat treatment deformation when the hardened layers are additionally formed on the shoulder portions 62, 63. In particular, since the wall thickness of the outer member 51 is reduced due to a pursuit to reduce weight, it is necessary to strictly ascertain whether or not the hardened layers should be formed on the shoulder portions 62, 63 in view of the strength, accuracy and manufacturing cost of the wheel bearing apparatus.